[FS#95] system looses IPv6 prefix after valid_lft reaches 0

[LEDE Bugs]

The following task has a new comment added:

FS#95 - system looses IPv6 prefix after valid_lft reaches 0
User who did this - satlan (satlan)

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I can confirm it's happening on my current LEDE build (WNDR3700). I can't pinpoint exactly when it started however.

And the above workaround worked. I've made it permanent for now.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=95#comment1011